The world is currently facing the challenge of global warming due to high levels of greenhouse gases particularly carbon dioxide emitted to the atmosphere. The key challenge is on reducing CO2 emissions at low cost. Several technologies are being investigated and applied for the capture and separation of carbon dioxide from flue gas streams including solvent, chemical looping, oxy-combustion, sorbent and membrane. Each of these technologies has areas best suited for application.
The method to remove acid gases from a gas stream using solvent is a well-known art. Typical absorbent solutions such as amine, carbonate, ammonia or amino acid salt solutions are used to remove the acid gas. A simplified conventional layout of such an absorption plant includes the use of an absorber and a desorber where the solution is circulated in a continuous cycle where the absorbent liquid is contacted countercurrent with the upward flowing gas. A main issue with these processes, especially in cases of removal of CO2 from low partial pressure flue gases, is the energy required for regenerating the absorbent in the desorber. Several technologies using the amine-based process have been developed including the conventional technology like the Fluor Econamine process which has been further improved to the Fluor Econamine FC Plus℠ process. Other amine base processes include MHI/KEPCO's KS-1, 2, and 3 (sterically hindered amines); Cansolv® Absorbent DC101™ (tertiary amines with a promoter); HTC Purenergy's mixed amine solvent and IFP's DMX. Promoted and un-promoted carbonate based process such as the Benfield process has also been developed and used mostly in the areas with medium to high pressures. An amino acid salt based process, The Alkazid process has earlier been developed developed by I.G. Farbenindustrie to remove H2S or mixtures of H2S and CO2 and more recently, amino acid salt technology has also been developed by Siemens. A common challenge in all the existing processes, in particular the low pressure processes, is the high regeneration energy requirement. For absorption technology to be a viable alternative for CO2 removal from exhaust gas stream, remarkable reductions in energy requirement for such technology in particular for CO2 removal from post combustion exhaust gases are needed.
Several patents and patent applications related to the technology where amine, carbonate or amino acid based absorbents are used for acidic gas removal have been disclosed.
U.S. Pat. No. 1,990,217 teaches the use of a salt of an organic acid and an inorganic base for gas purification, while U.S. Pat. No. 2,176,441 teaches the use of a salt of amino acids and inorganic or organic bases whereby the amino acid should be derived from a primary, secondary or tertiary amine having at least two nitrogen atoms. US 2007/0264180 A1 discloses use of an absorbent solution comprising compounds capable of forming two separable phases by addition of an acid that is stronger than the acid compounds of the gaseous effluent to be treated: a first phase rich in acid compounds and a second phase poor in acid compounds wherein amine, amino acids or amino-acid alkaline salts are used as activators to favour absorption of compounds to be eliminated. The process includes acid neutralization of multiamines to form two phase liquid and a process for separation of the two phase liquid. AU-B-67247/81 describes the use of an aqueous scrubbing solution comprising a mixture of a basic salt, potassium carbonate, and an activator for the said basic salt comprising at least one, sterically hindered amine and a sterically hindered amino acid as a cosolvent for the sterically hindered amine. The amino acid here serves to prevent two phase separation of the aqueous solution at high temperatures.
BE 767,105 discloses a process for removing acid gases from gaseous streams by contacting the gaseous streams with a solution comprising potassium carbonate and an amino acid.
These above-described prior processes involve improvements of the traditional absorption/desorption cycle using liquid absorbents throughout the process.
WO 2012/030630 A1 teaches the use of a system comprising an amine and/or amino acid salt capable of absorbing the CO2 and/or SO2 to produce a CO2- and/or SO2-containing solution; an amine regenerator to regenerate the amine and/or amino acid salt; and, when the system captures CO2, an alkali metal carbonate regenerator comprising an ammonium catalyst capable of catalyzing the aqueous alkali metal bicarbonate into the alkali metal carbonate and CO2 gas. This disclosure does not allow precipitation in the absorber. Precipitation is only allowed outside the absorber in the amine regenerator unit where a concentrated alkali carbonate is used to regenerate the amine/amino acid salts while producing alkali bicarbonate precipitate.
Solid precipitation in the absorber is typically avoided in most processes for CO2 removal, mainly because of potential plugging problems. WO 03/095071 A1 discloses a concept for the use of slurries from precipitating amino acid salt for CO2 capture. U.S. 2010/0092359 A1 discloses a method for capturing CO2 from exhaust gas in an absorber, wherein the CO2 containing gas is passed through an aqueous absorbent slurry. The aqueous absorbent slurry comprises an inorganic alkali carbonate, bicarbonate and at least one of an absorption promoter and a catalyst. CO2 is converted to solids by precipitation in the absorber. Said slurry having the precipitated solids is conveyed to a separating device, in which the solids are separated off, essentially all of at least one of the absorption promoter and catalyst is recycled together with the remaining aqueous phase to the absorber.
WO 2013/053853 A1 discloses a method for regeneration of bicarbonate slurry formed in a carbonate process.
The object of the present invention is to provide an absorbent system and method that improves carbon dioxide removal efficiency when compared with conventional amine, absorbent from organic acid neutralized with inorganic base carbonate based and ammonia absorbent systems.